Fish's food is typically composed of PLANKTON, which are small organisms that live in the ocean and other bodies of water. PLANKTON are an important part of the aquatic food chain, as they are a primary food source for many species of fish. PLANKTON are made up of tiny organisms such as algae, bacteria, protozoa, and zooplankton, which are all essential to the health of the marine environment. PLANKTON also provide a valuable source of nutrition for fish, as they are rich in proteins, vitamins, and minerals.